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Characterization Methods for FlexibleAbsorber Sheets WE-FAS

Application Note

ANP059 // JORGE VICTORIA AHUIR

1 The Flexible Absorber Sheet WE-FASElectromagnetic Interference (EMI) has become a serious problem as itcan occur anywhere in electronic circuits with unpredictable anddetrimental effects. This issue has grown in recent years due to a numberof factors including increases in device frequency, high integration inelectronic systems, higher power densities and reductions in PCBthickness and size.

The most common means of solving electromagnetic noise problems is toshield the system with conductive materials such as shielded enclosures,foil tape or conductive gaskets. Nevertheless, a large number of electronicdevices have many parts that operate at high frequencies that can causecomplex EMI problems that cannot be removed with conductive shielding.In order to avoid these issues, Flexible Absorber Sheets, like the WE-FASseries, which is made of a polymer filled with ferrite powder material, canbe used to suppress the unwanted high-frequency EM components.

Figure 1: WE-FAS Flexible Absorber Sheets

2 Absorber Sheet PropertiesOne of the most important parameters that describes the material’s abilityto absorb direct and dissipated electromagnetic noise is represented bythe part µ’’ of the complex permeability (µ). A material’s permeability isthe result of molecular composition and structure and is defined as thecomplex permeability. The real part quantifies the stored energy orinductive component while the imaginary part quantifies the absorbedenergy or absorption component:

µr = µr'- jµr" (Eq.1)

The behavior of these parameters depends on the material compositionand is frequency dependent. Thus, it is very important to know in whichfrequency range noise levels exceed maximum permitted limits. With thisdata, it is possible to establish a balance between reflection and magneticlosses depending on the application and the kind of electromagneticnoise. Figure 2 shows the complex permeability of several flexibleabsorber sheets of WE FAS series with different performancecharacteristics.

Figure 2: Permeability of flexible absorber sheet materials

As permeability is frequency dependent, the correct material need to beselected according to the frequency range where noise levels need to besuppressed. Standard specification don´t give this information. As theyonly state general parameters instead of absorption and reflectioncomponents:

Part number Thickness(mm) µ’typ @ 1MHz Dimensions

(mm)304 03S 0.3 23 330 x 210304 05S 0.5 23 330 x 210304 10S 1.0 23 330 x 210314 01 0.1 25 297 x 210314 02 0.2 25 297 x 210314 03 0.3 25 297 x 210324 01S 0.1 39 297 x 210324 02S 0.2 39 297 x 210324 03S 0.3 39 297 x 210324 05S 0.5 39 297 x 210324 075 S 0.75 39 297 x 210324 10S 1.0 39 297 x 210334 01 0.1 55 297 x 210334 02 0.2 55 297 x 210334 03 0.3 55 297 x 210344 01 0.1 100 297 x 210344 02 0.2 100 297 x 210344 03 0.3 100 297 x 210

Table 1: Overview of WE-FAS series

0

5

10

15

20

25

30

0

20

40

60

80

100

120

1 MHz 10 MHz 100 MHz 1 GHz

Abso

rptio

nµ''

'µ 304 'µ 314 'µ 324 'µ 334 'µ 344

''µ 304 ''µ 314 ''µ 324 ''µ 324 ''µ 344

Refle

ctio

nµ'

Frequency

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Characterization Methods for FlexibleAbsorber Sheets WE-FAS

Application Note

Nevertheless, it can be difficult to estimate the absorber materialperformance as it is the result of a multitude of variables in addition to theabsorber permeability. These include sheet thickness, size and geometry,as well as the distance between the noise source and absorber material.Thus, in basic systems the attenuation of a certain electromagnetic noisesuppression material cannot be estimated. However, in order to study theeffect in more complex electronic systems, it is often better to obtain realresults through some experimental characterization techniques.

It is more interesting to measure the absorbing capacity throughexperimental setups that make it possible to evaluate the materialperformance of several sheets that demonstrate different behavior.Thereby, several experimental tests are described that can be used tocharacterize absorber materials based upon internal and externalproperties.

The characterization techniques described simulate specific problemsfocused on transmission lines, cavity resonance and magnetic decoupling.These setups and experimental results will be shown below in order todetermine which material provides the best performance to reduceelectromagnetic noise problems depending on the application.

3 Microstrip Line MethodThis technique makes it possible to evaluate the performance of theflexible absorber sheets in systems with transmission line issues throughan experimental procedure. In this way, several sheets with differentcomposition or thickness can be tested in order to obtain the maximumtransmission attenuation power ratio in a specific application.

These problems can appear in high-frequency data buses where digitalsignals switch in the frequency range of MHz or GHz that can produceconducted noise on the data lines. An interesting solution in this sort ofapplication is to place an absorber sheet on the data bus as shown inFigure 3. This acts as a low-pass filter absorbing or attenuating highfrequency conducted noise.

Figure 3: Transmission Line Application

A method based on a microstrip line (MSL) test fixture is used to evaluatethe attenuation of conducting current noise in a PCB or noise path whenthe noise suppression sheet is in place. Thereby, the MSL is employed asa transmission line, whereby a noise signal will be measured to know thesample absorption ability. Hence, this test fixture simulates a noise sourceinside an electronic circuit and it is therefore possible to determine thetransmission absorption.

The manufactured MSL employed in this procedure consists of a PCBwhere the strip conductor is printed and two SMA type connectors areconnected in both ends. The MSL is composed of polytetrafluoroethylene(PTFE) dielectric PCB material (length = 100 mm, width = 50 mm,thickness = 1.6 mm), a copper strip conductor (length = 54.4 mm,width = 4.4 mm, thickness = 0.018 mm) and a copper ground planelocated in the bottom (length = 100 mm, width = 50 mm, thickness =0.018 mm). The SMA connectors are installed on the opposite side of theMSL and they are connected with the end of the MSL through two vias.

The absorbing ratio can be obtained by comparing the transmission linepower ratio before and after installing the absorbing sheet on the testfixture. In order to carry out the measurements, each end of a networkanalyzer coaxial cable is connected to each SMA test fixture ports, asshown in Figure 4. The network analyzer has to be configured to operateas signal source and signal receiver through measuring the S21parameters to start the measurement procedure.

Figure 4: Microstrip Line Measurement System

The results presented in Figure 5 show all available materials in 0.3 mmthickness which have been measured with the microstrip line setupdescribed above.

Figure 5: Transmission coefficient as a function of different materials

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0

0 GHz 1 GHz 2 GHz 3 GHz 4 GHz 5 GHz

30403 31403 32403 33403 34403

Tran

smiss

ionLo

ss(d

B)

Frequency

WE-FAS 0.3mm

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Characterization Methods for FlexibleAbsorber Sheets WE-FAS

Application Note

In the Appendix I you will find further data acquired of different materialswith different thickness of the WE-FAS range. Considering this, it ispossible to evaluate the absorber materials performance according to theircomposition and thickness.

4 Coaxial Line MethodCoaxial line experimental method provides a means of studying the noisesuppression capabilities of the material to solve resonant cavity EMIissues. Cavity resonance is a problem that can appear when an electroniccircuit is placed inside a metal enclosure. Noisy circuits usually cause theresonance inside the enclosure, which may cause interference problemsor even generate a system malfunction. Considering this, after evaluatingsome absorber materials with several compositions and sheet thickness,it is possible to choose the sheet with the best performance to filter theresonant frequency. In this kind of application, a sheet is placed under thecover of the metal enclosure to absorb the electromagnetic noise asillustrated in Figure 6. A means of reducing these issues is to place anabsorber sheet inside the enclosure to attenuate or suppress theresonance by reducing the internal reflections. To evaluate this materialin this kind of application, an experimental measurement system basedon a coaxial line is utilized. In this procedure, one terminal of the coaxialline is shorted with a metallic surface, and the reflected energy inside it ismeasured with a network analyzer, as shown in Figure 7.

In order to characterize the absorption capacity of different compositionsand/or thicknesses, it is necessary to repeat the process setting theabsorber material attached over the reflector and compare the results.The evaluation of absorber materials in this experiment is carried out bymeasuring the reflection parameter (S11) with the reference value set asthe coaxial line without the absorber material. Next, the sample is placedbetween the reflection material and a reflective material before themeasurement is made. By comparing reference and measured data, it ispossible to study the performance of each sheet. The reflectionattenuation measured with the experimental coaxial line method as shownin Figure 8 with 0.3 mm thickness. Again, several

WE-FAS with different materials and thicknesses have been tested in theAppendix II in order to evaluate the attenuation ability in each case.

Figure 6: Coaxial Line Application

Figure 7: Coaxial Line Measurement Procedure

Abbildung 8: Reflexionsdämpfung als Funktion unterschiedlicherMaterialien

5 Magnetic Decoupling MethodMagnetic decoupling is a common issue in electronic systems, whicheffects PCBs or devices with high-density layouts and circuits. Thiselectromagnetic interference situation is often observed when there is anoisy electronic component or circuit placed on a PCB that can affectnearby elements and components. An interesting solution which canreduce the unintentional coupling of this kind is to focus on filtering theelectromagnetic interference by placing the absorber sheet on the noisesource or protecting the interfered part, as shown in Figure 9.

This technique analyzes the magnetic sheets absorption capacity forreducing or suppressing the coupling by employing a microstrip line testfixture to generate the EMI interference and a near field probe (NFP) tomeasure them. Thereby, this setup simulates a coupling system wherebyone is used as a noise source and the other as a receiver or victim. Theabsorption effect is determined through connecting one port of thenetwork analyzer to one terminal of the microstrip line and terminating the

perpendicular to the track and connected to the second port of thenetwork analyzer, as shown in Figure 10. Subsequently, a networkanalyzer is configured as the signal source and signal receiver and theS21 parameters are compared before (reference) and after placing eachabsorber sheet on the microstrip line. The measured data with theabsorber sample in place is subtracted from the reference.

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0

0 GHz 1 GHz 2 GHz 3 GHz

30403 31403 32403 33403 34403

Refle

ction

Loss

(dB)

Frequency

WE-FAS 0.3 mm

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Characterization Methods for FlexibleAbsorber Sheets WE-FAS

Application Note

Figure 10: Magnetic Decoupling Measurement Technique

Data obtained through the magnetic decoupling technique is representedin Figure 11. The graph chart shows the measurement of WE-FAS withdifferent material in 0.3 mm thickness. More graph charts in theAppendix III show the data also for different thicknesses.

Figure 11: Magnetic Decoupling as a function of sheet materials

6 SummaryTaking everything into consideration, WE-FAS Flexible Absorber Sheetscan solve a variety of EMI problems in different kinds of applications.Thereby, absorber materials have been characterized in order to beemployed in applications with data buses affected by conductedproblems, electronic circuits located inside an enclosure withmalfunctions due to cavity resonance and electromagnetic interferencebetween components placed in the same circuit or in close proximity to it.

Absorber sheets can provide several advantages in controlling EMI. Thisis confirmed with experimental techniques, which provide results in termsof attenuation ratio depending on the kind of problem thereforedemonstrating the best option to use when solving a specific EMI problem.Furthermore, several thicknesses have been evaluated to demonstrate theperformance of different products depending on the device space limits.Therefore, these materials can provide an innovative, straightforwardsolution without the need to modify or redesign the electronic circuit orproduct.

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0.0 GHz 0.2 GHz 0.4 GHz 0.6 GHz 0.8 GHz 1.0 GHz

30403 31403 32403 33403 34403

Mag

netic

Deco

uplin

g(dB

)

Frequency

WE-FAS 0.3 mm

Figure 9: Magnetic Decoupling Applications

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Characterization Methods for FlexibleAbsorber Sheets WE-FAS

Application Note

A. AppendixA.1. Appendix I - Transmission coefficient as a function of sheet thickness

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0 GHz 1 GHz 2 GHz 3 GHz 4 GHz 5 GHz

0.1 mm 0.3 mm 0.5 mm 0.75 mm 1.0 mm

Tran

smiss

ionLo

ss(d

B)

Frequency

WE-FAS 304

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0

0 GHz 1 GHz 2 GHz 3 GHz 4 GHz 5 GHz

0.1 mm 0.2 mm 0.3 mm

Tran

smiss

ionLo

ss(d

B)Frequency

WE-FAS 314

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0

0 GHz 1 GHz 2 GHz 3 GHz 4 GHz 5 GHz

0.1 mm 0.3 mm 0.5 mm 0.75 mm 1.0 mm

Tran

smiss

ionLo

ss(d

B)

Frequency

WE-FAS 324

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0 GHz 1 GHz 2 GHz 3 GHz 4 GHz 5 GHz

0.1 mm 0.2 mm 0.3 mm

Tran

smiss

ionLo

ss(d

B)

Frequency

WE-FAS 334

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0

0 GHz 1 GHz 2 GHz 3 GHz 4 GHz 5 GHz

0.1 mm 0.2 mm 0.3 mm

Tran

smiss

ionLo

ss(d

B)

Frequency

WE-FAS 344

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Characterization Methods for FlexibleAbsorber Sheets WE-FAS

Application Note

A.2. Appendix II - Reflection coefficient as a function of sheet thickness

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-3

-2

-1

0

0 GHz 1 GHz 2 GHz 3 GHz

0.1 mm 0.3 mm 0.5 mm 0.75 mm 1.0 mm

Refle

ction

Loss

(dB)

Frequency

WE-FAS 304

-7

-6

-5

-4

-3

-2

-1

0

0 GHz 1 GHz 2 GHz 3 GHz

0.1 mm 0.2 mm 0.3 mmRe

flect

ionLo

ss(d

B)Frequency

WE-FAS 314

-7

-6

-5

-4

-3

-2

-1

0

0 GHz 1 GHz 2 GHz 3 GHz

0.1 mm 0.2 mm 0.3 mm 0.5 mm 1.0 mm

Refle

ction

Loss

(dB)

Frequency

WE-FAS 324

-7

-6

-5

-4

-3

-2

-1

0

0 GHz 1 GHz 2 GHz 3 GHz

0.1 mm 0.2 mm 0.3 mm

Refle

ction

Loss

(dB)

Frequency

WE-FAS 334

-7

-6

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-4

-3

-2

-1

0

0 GHz 1 GHz 2 GHz 3 GHz

0.1 mm 0.2 mm 0.3 mm

Refle

ction

Loss

(dB)

Frequency

WE-FAS 344

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Characterization Methods for FlexibleAbsorber Sheets WE-FAS

Application Note

A.3. Appendix III - Magnetic Decoupling as a function of sheet thickness

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0.1 mm 0.3 mm 0.5 mm 0.75 mm 1.0 mm

Mag

netic

Deco

uplin

g(dB

)

Frequency

WE-FAS 304

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0

0.0 GHz 0.2 GHz 0.4 GHz 0.6 GHz 0.8 GHz 1.0 GHz

0.1 mm 0.2 mm 0.3 mmM

agne

ticDe

coup

ling(

dB)

Frequency

WE-FAS 314

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0

0.0 GHz 0.2 GHz 0.4 GHz 0.6 GHz 0.8 GHz 1.0 GHz

0.1 mm 0.2 mm 0.3 mm 0.5 mm 1.0 mm

Mag

netic

Deco

uplin

g(dB

)

Frequency

WE-FAS 324

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0

0.0 GHz 0.2 GHz 0.4 GHz 0.6 GHz 0.8 GHz 1.0 GHz

0.1 mm 0.2 mm 0.3 mm

Mag

netic

Deco

uplin

g(dB

)

Frequency

WE-FAS 334

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0.0 GHz 0.2 GHz 0.4 GHz 0.6 GHz 0.8 GHz 1.0 GHz

0.1 mm 0.2 mm 0.3 mm

Mag

netic

Deco

uplin

g(dB

)

Frequency

WE-FAS 344

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Characterization Methods for FlexibleAbsorber Sheets WE-FAS

Application Note

A.4. Bill of MaterialPart number Thickness (mm) µ’typ @ 1MHz Dimensions (mm)

304 03S 0.3 23 330 x 210

304 05S 0.5 23 330 x 210

304 10S 1.0 23 330 x 210

314 01 0.1 25 297 x 210

314 02 0.2 25 297 x 210

314 03 0.3 25 297 x 210

324 01S 0.1 39 297 x 210

324 02S 0.2 39 297 x 210

324 03S 0.3 39 297 x 210

324 05S 0.5 39 297 x 210

324 075 S 0.75 39 297 x 210

324 10S 1.0 39 297 x 210

334 01 0.1 55 297 x 210

334 02 0.2 55 297 x 210

334 03 0.3 55 297 x 210

344 01 0.1 100 297 x 210

344 02 0.2 100 297 x 210

344 03 0.3 100 297 x 210

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Characterization Methods for FlexibleAbsorber Sheets WE-FAS

Application Note

I M P O R T A N T N O T I C EThe Application Note is based on our knowledge and experience of typicalrequirements concerning these areas. It serves as general guidance andshould not be construed as a commitment for the suitability for customerapplications by Würth Elektronik eiSos GmbH & Co. KG. The information inthe Application Note is subject to change without notice. This documentand parts thereof must not be reproduced or copied without writtenpermission, and contents thereof must not be imparted to a third party norbe used for any unauthorized purpose.Würth Elektronik eiSos GmbH & Co. KG and its subsidiaries and affiliates(WE) are not liable for application assistance of any kind. Customers mayuse WE’s assistance and product recommendations for their applicationsand design. The responsibility for the applicability and use of WE Productsin a particular customer design is always solely within the authority of thecustomer. Due to this fact it is up to the customer to evaluate andinvestigate, where appropriate, and decide whether the device with thespecific product characteristics described in the product specification isvalid and suitable for the respective customer application or not.The technical specifications are stated in the current data sheet of theproducts. Therefore the customers shall use the data sheets and arecautioned to verify that data sheets are current. The current data sheetscan be downloaded at www.we-online.com. Customers shall strictlyobserve any product-specific notes, cautions and warnings. WE reservesthe right to make corrections, modifications, enhancements,improvements, and other changes to its products and services.WE DOES NOT WARRANT OR REPRESENT THAT ANY LICENSE, EITHEREXPRESS OR IMPLIED, IS GRANTED UNDER ANY PATENT RIGHT,

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